In the electrical power supply circuit of an electronic device and the like, a DC to DC converter of chopper type is widely used, which supplies to a load the direct current power output with a voltage different from that of a direct current power source (see Patent Documents 1 to 4, for example). The DC to DC converter of chopper type, at first, chops the direct current power from a direct current power source, by the direct switching operations of switching elements and converts it to high frequency electric power. The high frequency electric power is smoothed with a reactor and an output condenser, and is further converted into the direct current power again. More specifically, a DC to DC converter of step down chopper type is proposed, which includes a direct current power source, a transistor, an output diode, a reactor, an output condenser and a control circuit (see Patent Document 1, for example).
In the DC to DC converter of step down chopper type, the transistor works as a switching device, the collector terminal (one main terminal) of which is connected to the positive terminal (one end) of the direct current power source. The output diode works as an output rectifying device of feedback use, which is connected to the emitter terminal (the other main terminal) of the transistor and the negative terminal (the other end) of the direct current power source. The reactor has an end connected to the connection point of the transistor and the output diode. The output condenser is connected to the other end of the reactor and also to the negative terminal of the direct current power source. A load is connected with the output condenser, in parallel to each other. The control circuit sends a control pulse signal to the base terminal of the transistor and achieves the on and off control of the transistor.
The DC to DC converter of step down chopper type is capable of supplying to a load the direct current power output with a voltage lower than that of a direct current power source, by controlling the on and off action of a transistor. When the transistor turned on or turned off, switching loss, based on the overlapping portion between a collector to emitter voltage waveform (VCE) of the transistor and a collector current waveform (IC) of the transistor, is generated in a large quantity. The collector to emitter voltage waveform (VCE) of the transistor and the collector current waveform (IC) of the transistor are steep at a leading edge of the waveform. Accordingly, surges in voltage (Vsr), surges in current (Isr) and noises are produced in a spike like manner.
In order to reduce the before-mentioned surges and noises, a DC to DC converter of chopper type has been proposed, which includes a direct current power source, a switching device, an output rectifying device, a reactor, an output condenser, a resonance reactor, a first rectifying device, a first resonance condenser, a second rectifying device, a second resonance condenser, a third rectifying device (see Patent Document 1, for example). A load is connected with the output condenser, in parallel to each other. The direct current power source is composed of a rectifying circuit which converts the alternative current voltage of an alternative current power source into the direct current voltage. The switching device has one main terminal which is connected to one end of the direct current power source. The output rectifying device is connected to the other main terminal of the switching device and also to the other end of the direct current power source. The reactor has one end which is connected to the connection point of the switching device and the output rectifying device.
The output condenser is connected to the other end of the reactor and also to the other end of the direct current power source. The resonance reactor is connected to the switching device and also to the connection point of the output rectifying device and the reactor. The first rectifying device has one end which is connected to the connection point of the switching device and the resonance reactor. The first resonance condenser has one end which is connected to the connection point of the resonance reactor and the output rectifying device. The second rectifying device is connected to the other end of the first resonance condenser and also to the other end of the direct current power source. The second resonance condenser is connected to the other end of the first rectifying device and also to one end of the direct current power source. The third rectifying device is connected to the other end of the first rectifying device and also to the other end of the first resonance condenser.
The DC to DC converter of chopper type supplies the direct current power output with a voltage lower than that of a direct current power source to the load, by turning the switching device on and off. When the switching device is on an off-state, the first resonance condenser is discharged. At the same time, the second resonance condenser is charged in a sine wave manner. Moreover, when the switching device is turned on, the second resonance condenser is discharged. Thereby, the first resonance condenser and the second resonance condenser and the resonance reactor are made into resonance state and the resonance current flows into the switching device. When the switching device is turned off, the first rectifying device is turned into a forward bias state. The electric current which flows through the switching device is switched promptly into an electric current which flows through the second resonance condenser.
The first resonance condenser is discharged, and in addition, the second resonance condenser is charged in a sine wave manner. Accordingly, the voltage between both ends of the switching device starts to rise in a sine wave manner from 0 V, and thereby, a zero voltage switching is achieved when the switching device is on a turn-off state. The switching device produces a reduced switching loss, at the time of turn off. When the switching device becomes the on-state from the off-state, the second resonance condenser is discharged. The first resonance condenser and the second resonance condenser and the resonance reactor enter into the resonant state, and the resonant current flows through the switching device.
The electric current which flows through the switching device increases linearly from zero. Accordingly, the zero current switching is achieved at the time when the switching device is on a turn-on state. Thereby, the switching loss at the time of turn-on of the switching device can be reduced. By the resonance actions among the first resonance condenser, the second resonance condenser and the resonance reactor, the switching loss at the time of on and off actions of the switching device is lowered, and in addition, the spike like surged voltage and surged current are also lowered,
Moreover, the electric current which flows through the output rectifying device gradually decreases, by the self-induction mechanism of the resonance reactor at the time when the switching device is on a turn-on state. The recovery current, which flows through the output rectifying device in a reverse direction, decreases at the time when the switching device is on a turn-on state. In consequence, a current limiting reactor can be eliminated from the power supply circuit and the number of components can be reduced. In addition, the switching loss and noises due to the recovery characteristic features of the output rectifying device can be further reduced at the time when the switching device is on a turn-on state.